The material in this article is intended not only for owners of already rare televisions who want to restore their functionality, but also for those who want to understand the circuitry, structure and operating principle of switching power supplies. If you master the material in this article, you can easily understand any circuit and operating principle of switching power supplies for household appliances, be it a TV, laptop or office equipment. And so let's get started...
Soviet-made televisions, the third generation ZUSTST, used switching power supplies - MP (power module).
Switching power supplies, depending on the TV model where they were used, were divided into three modifications - MP-1, MP-2 and MP-3-3. The power modules are assembled according to the same electrical circuit and differ only in the type of pulse transformer and the voltage rating of capacitor C27 at the output of the rectifier filter (see circuit diagram).
Rice. 1. Functional diagram of the switching power supply for the ZUSTST TV:
1 - network rectifier; 2 - trigger pulse generator; 3 - pulse generator transistor, 4 - control cascade; 5 - stabilization device; 6 - protection device; 7 - pulse transformer of the TV power supply 3ust; 8 - rectifier; 9 - load
Let at the initial moment of time a pulse be generated in device 2, which will open the transistor of the pulse generator 3. At the same time, a linearly increasing sawtooth current will begin to flow through the winding of the pulse transformer with pins 19, 1. At the same time, energy will accumulate in the magnetic field of the transformer core, the value of which is determined by the open time of the pulse generator transistor. The secondary winding (pins 6, 12) of the pulse transformer is wound and connected in such a way that during the period of magnetic energy accumulation, a negative potential is applied to the anode of the VD diode and it is closed. After some time, control cascade 4 closes the pulse generator transistor. Since the current in the winding of transformer 7 cannot change instantly due to the accumulated magnetic energy, a self-induction emf of the opposite sign occurs. The VD diode opens, and the secondary winding current (pins 6, 12) increases sharply. Thus, if in the initial period of time the magnetic field was associated with the current that flowed through winding 1, 19, now it is created by the current of winding 6, 12. When all the energy accumulated during the closed state of switch 3 goes into the load, then in the secondary winding will reach zero.
From the above example we can conclude that by adjusting the duration of the open state of the transistor in a pulse generator, you can control the amount of energy that goes to the load. This adjustment is carried out using control cascade 4 using a feedback signal - the voltage at the terminals of winding 7, 13 of the pulse transformer. The feedback signal at the terminals of this winding is proportional to the voltage across the load 9.
If the voltage across the load decreases for some reason, the voltage supplied to the stabilization device 5 will also decrease. In turn, the stabilization device, through the control cascade, will begin to close the pulse generator transistor later. This will increase the time during which current will flow through winding 1, 19, and the amount of energy transferred to the load will accordingly increase.
The moment of the next opening of transistor 3 is determined by the stabilization device, where the signal coming from winding 13, 7 is analyzed, which allows you to automatically maintain the average value of the output DC voltage.
The use of a pulse transformer makes it possible to obtain voltages of different amplitudes in the windings and eliminates the galvanic connection between the circuits of secondary rectified voltages and the supply electrical network. Control stage 4 determines the range of pulses created by the generator and, if necessary, turns it off. The generator is switched off when the mains voltage drops below 150 V and the power consumption drops to 20 W, when the stabilization cascade stops functioning. When the stabilization cascade is not working, the pulse generator becomes uncontrollable, which can lead to the appearance of large current pulses in it and to the failure of the pulse generator transistor.
Let's look at the circuit diagram of the MP-3-3 power module and the principle of its operation.
Rice. 2 Schematic diagram of a switching power supply for a ZUSTST TV, module MP-3-3
It includes a low-voltage rectifier (diodes VD4 - VD7), a trigger pulse shaper (VT3), a pulse generator (VT4), a stabilization device (VT1), a protection device (VT2), a pulse transformer T1 of the 3ustst power supply and rectifiers using diodes VD12 - VD15 with voltage stabilizer (VT5 - VT7).
The pulse generator is assembled according to a blocking generator circuit with collector-base connections on a VT4 transistor. When you turn on the TV, the constant voltage from the output of the low-voltage rectifier filter (capacitors C16, C19 and C20) through winding 19, 1 of transformer T1 is supplied to the collector of transistor VT4. At the same time, the mains voltage from diode VD7 through capacitors C11, C10 and resistor R11 charges capacitor C7, and also goes to the base of transistor VT2, where it is used in the device for protecting the power module from low voltage. When the voltage on capacitor C7 applied between the emitter and base 1 of unijunction transistor VT3 reaches 3 V, transistor VT3 will open. Capacitor C7 is discharged through the circuit: emitter-base junction 1 of transistor VT3, emitter junction of transistor VT4, parallel connected, resistors R14 and R16, capacitor C7.
The discharge current of capacitor C7 opens transistor VT4 for a time of 10 - 15 μs, sufficient for the current in its collector circuit to increase to 3...4 A. The flow of collector current of transistor VT4 through the magnetization winding 19, 1 is accompanied by the accumulation of energy in the magnetic field of the core. After capacitor C7 has finished discharging, transistor VT4 closes. The cessation of the collector current causes the appearance of a self-induction EMF in the coils of transformer T1, which creates positive voltages at terminals 6, 8, 10, 5 and 7 of transformer T1. In this case, current flows through the diodes of half-wave rectifiers in the secondary circuits (VD12 - VD15).
With a positive voltage at terminals 5, 7 of transformer T1, capacitors C14 and C6 are charged, respectively, in the anode and control electrode circuits of thyristor VS1 and C2 in the emitter-base circuit of transistor VT1.
Capacitor C6 is charged through the circuit: pin 5 of transformer T1, diode VD11, resistor R19, capacitor C6, diode VD9, pin 3 of the transformer. Capacitor C14 is charged through the circuit: pin 5 of transformer T1, diode VD8, capacitor C14, pin 3 of transformer. Capacitor C2 is charged through the circuit: pin 7 of transformer T1, resistor R13, diode VD2, capacitor C2, pin 13 of the transformer.
The subsequent switching on and off of the blocking generator transistor VT4 is carried out similarly. Moreover, several such forced oscillations are sufficient to charge the capacitors in the secondary circuits. With the completion of charging of these capacitors, positive feedback begins to operate between the windings of the blocking generator connected to the collector (pins 1, 19) and the base (pins 3, 5) of the VT4 transistor. In this case, the blocking generator goes into self-oscillation mode, in which transistor VT4 will automatically open and close at a certain frequency.
During the open state of transistor VT4, its collector current flows from the plus of electrolytic capacitor C16 through the winding of transformer T1 with terminals 19, 1, the collector and emitter junctions of transistor VT4, parallel connected resistors R14, R16 to the minus of capacitor C16. Due to the presence of inductance in the circuit, the collector current increases according to a sawtooth law.
To eliminate the possibility of failure of transistor VT4 from overload, the resistance of resistors R14 and R16 is selected in such a way that when the collector current reaches 3.5 A, a voltage drop is created across them sufficient to open thyristor VS1. When the thyristor opens, capacitor C14 is discharged through the emitter junction of transistor VT4, resistors R14 and R16 connected in parallel, and open thyristor VS1. The discharge current of capacitor C14 is subtracted from the base current of transistor VT4, which leads to its premature closing.
Further processes in the operation of the blocking generator are determined by the state of the thyristor VS1, the earlier or later opening of which allows you to regulate the rise time of the sawtooth current and thereby the amount of energy stored in the transformer core.
The power module can operate in stabilization and short circuit mode.
The stabilization mode is determined by the operation of the DC amplifier (DC amplifier) assembled on transistor VT1 and thyristor VS1.
At a network voltage of 220 Volts, when the output voltages of the secondary power supplies reach rated values, the voltage on the winding of transformer T1 (pins 7, 13) increases to a value at which the constant voltage at the base of the transistor VT1, where it is supplied through the divider Rl - R3, becomes more negative than at the emitter, where it is completely transmitted. Transistor VT1 opens along the circuit: pin 7 of the transformer, R13, VD2, VD1, emitter and collector junctions of transistor VT1, R6, control electrode of the thyristor VS1, R14, R16, pin 13 of the transformer. This current, summed with the initial current of the control electrode of the thyristor VS1, opens it at the moment when the output voltage of the module reaches the nominal values, stopping the increase in the collector current.
By changing the voltage at the base of transistor VT1 with trimming resistor R2, you can adjust the voltage across resistor R10 and, therefore, change the opening moment of thyristor VS1 and the duration of the open state of transistor VT4, thereby setting the output voltage of the power supply.
When the load decreases (or the network voltage increases), the voltage at terminals 7, 13 of transformer T1 increases. At the same time, the negative voltage at the base increases in relation to the emitter of transistor VT1, causing an increase in the collector current and a voltage drop across resistor R10. This leads to earlier opening of thyristor VS1 and closing of transistor VT4. This reduces the power supplied to the load.
When the network voltage decreases, the voltage on the winding of transformer T1 and the base potential of transistor VT1 relative to the emitter become correspondingly lower. Now, due to a decrease in the voltage created by the collector current of transistor VT1 on resistor R10, thyristor VS1 opens at a later time and the amount of energy transferred to the secondary circuits increases. An important role in protecting transistor VT4 is played by the cascade on transistor VT2. When the network voltage decreases below 150 V, the voltage on the winding of transformer T1 with terminals 7, 13 is insufficient to open transistor VT1. In this case, the stabilization and protection device does not work, transistor VT4 becomes uncontrollable and the possibility of its failure is created due to exceeding the maximum permissible values of voltage, temperature, and current of the transistor. To prevent the failure of transistor VT4, it is necessary to block the operation of the blocking generator. The transistor VT2 intended for this purpose is connected in such a way that a constant voltage is supplied to its base from the divider R18, R4, and a pulsating voltage with a frequency of 50 Hz is supplied to the emitter, the amplitude of which is stabilized by the zener diode VD3. When the network voltage decreases, the voltage at the base of transistor VT2 decreases. Since the voltage at the emitter is stabilized, a decrease in the voltage at the base causes the transistor to open. Through the open transistor VT2, trapezoidal-shaped pulses from the diode VD7 arrive at the control electrode of the thyristor, opening it for a time determined by the duration of the trapezoidal pulse. This causes the blocking generator to stop working.
Short circuit mode occurs when there is a short circuit in the load of secondary power supplies. In this case, the power supply is started by triggering pulses from the trigger device assembled on transistor VT3, and turned off using thyristor VS1 according to the maximum collector current of transistor VT4. After the end of the triggering pulse, the device is not excited, since all the energy is spent in the short-circuited circuit.
After the short circuit is removed, the module enters stabilization mode.
Pulse voltage rectifiers connected to the secondary winding of transformer T1 are assembled using a half-wave circuit.
The VD12 diode rectifier creates a voltage of 130 V to power the horizontal scanning circuit. The ripples of this voltage are smoothed out by the electrolytic capacitor C27. Resistor R22 eliminates the possibility of a significant increase in voltage at the rectifier output when the load is turned off.
A 28 V rectifier is assembled on the VD13 diode, designed to power the vertical scanning of a TV. Voltage filtering is provided by capacitor C28 and inductor L2.
A 15 V voltage rectifier for powering an audio amplifier is assembled using a VD15 diode and a SZO capacitor.
The 12 V voltage used in the color module (MC), radio channel module (MRK) and vertical scanning module (MS) is created by a rectifier based on diode VD14 and capacitor C29. At the output of this rectifier, a compensation voltage regulator assembled on transistors is included. It consists of a regulating transistor VT5, a current amplifier VT6 and a control transistor VT7. The voltage from the output of the stabilizer through the divider R26, R27 is supplied to the base of the transistor VT7. Variable resistor R27 is designed to set the output voltage. In the emitter circuit of transistor VT7, the voltage at the output of the stabilizer is compared with the reference voltage at the zener diode VD16. The voltage from the collector VT7 through the amplifier on the transistor VT6 is supplied to the base of the transistor VT5, connected in series to the rectified current circuit. This leads to a change in its internal resistance, which, depending on whether the output voltage has increased or decreased, either increases or decreases. Capacitor C31 protects the stabilizer from excitation. Through resistor R23, voltage is supplied to the base of transistor VT7, which is necessary to open it when turned on and restore it after a short circuit. Choke L3 and capacitor C32 are an additional filter at the output of the stabilizer.
Capacitors C22 - C26 bypass rectifier diodes to reduce interference emitted by pulsed rectifiers into the electrical network.
The PFP power filter board is connected to the electrical network via connector X17 (A12), switch S1 in the TV control unit and mains fuses FU1 and FU2.
VPT-19 type fuses are used as mains fuses, the characteristics of which make it possible to provide significantly more reliable protection of television receivers in the event of malfunctions than PM type fuses.
The purpose of the barrier filter is .
On the power filter board there are barrier filter elements (C1, C2, SZ, inductor L1) (see circuit diagram).
Resistor R3 is designed to limit the current of the rectifier diodes when the TV is turned on. The posistor R1 and resistor R2 are elements of the kinescope mask demagnetization device.
Not bad Charger with good output characteristics can be made from old TVs with pulsed power supplies such as MP1, MP3-3, MP403, etc. Minor modification of the unit allows it to be used for charging battery with current up to 6-7A, repair of car radios and other equipment.
The whole point of remaking the block is to increase the load capacity of TPI and rectifier diodes, for this we connect windings with pins 12,18 and 10,20 in parallel, pin 20 is connected to the common pin of secondary sources (12), and pin 10 is connected to pin 18, rectifier diodes 12V and 15V turn it off and connect a diode with a current of 10-25A to pins 10, 18, which must be installed on a heat sink; for these purposes I used a heat sink from a standard 12 V stabilizer.
Details of which are unnecessary you can remove it from the board (except for the so-called outlet), you can put a new diode on it, connect a 470 pf capacitor in parallel to it and at the output electrolyte 470 uF x 40 V, parallel to it we put a load resistor MLT 2 with a nominal value of 510-680 ohms and a ceramic capacitor at 1 µF, these parts are installed to prevent the appearance of high-frequency voltage at the output of the power supply.
To adjust the output voltage You can use trimming resistor R2 according to the circuit, which is soldered off and instead of it we connect an external variable wire resistor of the PPZ type 1-1.5 kohm, adjusting the output voltage from 13V to 18V.
To put the block into mode To stabilize it, you need to load it; for this you can use a lamp from the refrigerator, connecting it to pins 6 and 18.
In your loading block I used the +28 V output, connecting to it a 28 V 5W lamp, which simultaneously serves as a backlight for the voltmeter scale with an extended scale from “five”. The unit heats up under load as in normal mode, but it will be better if you make forced airflow by installing a cooler from the computer.
When connecting the battery, it is necessary to observe the polarity and install a 10A fuse at the output.
Chapter 3. Schemes of switching power supplies.
In this article we will consider a scheme in which key management is done according to a different principle. This scheme, with minor changes, is used in many TVs, such as Akai CT-1405E, Elekta CTR-2066DS and others.
A comparison device is assembled on transistor Q1; its circuit is no different from others discussed earlier. Only here an n-p-n transistor is used, as a result the switching polarity has changed. The comparison circuit is powered from a separate winding from the rectifier D5 with filter C2. The initial bias to switch Q4 is supplied through resistor R7, which is usually several resistors connected in series, which is apparently explained by better heat transfer, the elimination of breakdown between the terminals (after all, the voltage drop across it is 300 V) or the manufacturability of the assembly. I myself don’t know why this is done, but in imported equipment you see this all the time.
The feedback circuit is connected here in a different way than we discussed earlier. One terminal of the feedback winding is connected as usual, to the base of the key, and the other to the diode distributor D3, D4.
What is the result? Transistors Q2 and Q3, which are a composite transistor, are adjustable resistance. This resistance (between the positive of capacitor C3 and the emitter of Q3) depends on the error signal coming from Q1. Since transistor Q2 has p-n-p conductivity, with an increase in the voltage coming to its base, its current decreases, transistor Q3 closes, that is, the resistance of the composite transistor increases. This property of the circuit is used.
Let's consider the moment of launch. Capacitor C3 is discharged. The feedback circuit is connected by plus to the base, minus through D4 and R9 with a common wire. There is a process of linear increase in the collector current, which ends with the switch being saturated and closing. In this case, the polarity of the voltage on the feedback winding is reversed and this voltage charges capacitor C3 through diode D3. When the energy of the transformer is used up, capacitor C3 will be connected to the base-emitter junction of the switch through the resistance of the composite transistor with a minus to the base and closes the switch.
The discharge time of C3 and the value of the closing potential depend on the resistance value of the composite transistor. At the moment the power supply starts, this resistance is large and the discharge of capacitor C3 does not delay the next cycle, however, in steady state, the delay of the next cycle is sufficient to regulate the average power supplied to the load. Thus, we see that the circuit in question is not exactly PWM. If in previous schemes the time of the open state of the key was subject to regulation, then in this scheme the time of the closed state of the key is regulated.
Fig 2
The figure shows the discharge path of capacitor C3. At time t0, the switch collector current begins to increase and continues until time t1. During this period of time, the voltage Ube of the key increases. This does not affect the charge of C3 in any way, since C3 is connected to the feedback winding through diode D3, which is closed at this moment. As soon as the increase in the collector current of the switch ends, the polarity of the voltage on the feedback winding changes to reverse, diode D3 opens and charging C3 begins. At the same time, through the resistance of the composite transistor Rstate, this voltage is applied to the base-emitter junction of the switch, reliably locking it. Charge C3 continues until time t2, that is, until the accumulated energy of the transformer is transferred to the load. At this moment, charged C3 through Rstate and the opened diode D4 will be connected to the base-emitter junction of the switch. The figure below shows how the voltage of the charged capacitor C3 is divided between the resistance of the composite transistor Rcomp (Ucomp) and the resistance of the base-emitter section of the switch Rcl (Ube), which is determined by the sum of the resistances R9 and the resistance of the open diode D4. The resistance of resistors R6, R9 and R10 is small and can be ignored. With a high resistance Rstate, the discharge of C3 occurs more slowly and the threshold for opening the key will be reached later than with a low Rstate. At time t3, voltage C3 will decrease to such a value that the locking voltage at the base of the key will disappear and the cycle will repeat. So the resistance of the composite transistor participates in the process.
Schemes of domestic switching power supplies.
The vast majority of domestic UPS circuits are built according to the same circuit, according to the same principle, and differ only in the startup circuit and the output voltage values of the secondary rectifiers. And one more feature - domestic UPSs are not designed to operate in standby mode (that is, in almost idle mode). All UPSs have protection against overload and short circuit in the load, against undervoltage in the network below 160 V, and no-load. In some models with remote control, the UPS is turned off using an artificially created overload, in which case the overload protection is triggered and generation is disrupted.
Since there are still a lot of domestic TVs with such UPSs, I will talk about them in more detail, despite the fact that I will repeat myself in some areas. What I will talk about applies to all UPS models built on discrete elements. We will consider domestic UPSs built using the K1033EU1 microcircuit (analogous to TDA4601) in the next chapter, in which I will describe the operation of UPS on microcircuits. I will not consider newer UPSs that use developments from foreign manufacturers here.
Schematic diagram of the MP-3-3 power module
Let's look at the circuit diagram of the MP-3-3 power module. The module includes a low-voltage rectifier (diodes VD4-VD7), a trigger pulse shaper (VT3), a pulse generator (VT4), a stabilization device (VT1), a protection device (VT2), a pulse transformer T1, rectifiers on diodes VD12-VD15, a stabilizer voltage 12 V (VT5-VT7).
Fig 3
The pulse generator is assembled according to a self-oscillator circuit with collector-base connections on a VT4 transistor. When the TV is turned on, the constant voltage from the output of the mains rectifier filter (capacitors C16, C19, C20) through winding 19-1 of transformer T1 is supplied to the collector of transistor VT4. At the same time, the mains voltage from diode VD7 through resistors R8 and R 11 charges capacitor C7, and is also supplied to the emitter of transistor VT2, where it is used in the device for protecting the power module from low mains voltage. When the voltage across capacitor C7 applied between the emitter and base 1 of unijunction transistor VT3 reaches 3 V, transistor VT3 opens. Capacitor C7 begins to discharge along the circuit: emitter-base junction of transistor VT3, emitter junction of transistor VT4, parallel connected resistors R14 and R16, capacitor C7.
The discharge current of capacitor C7 opens transistor VT4 for a time of 10...15 μs, sufficient for the current in its collector circuit to increase to 3...4 A. The flow of the collector current of transistor VT4 through the magnetization winding 19-1 is accompanied by the accumulation of energy in the magnetic field core. After capacitor C7 has finished discharging, transistor VT4 closes. The cessation of the collector current causes the appearance of a self-induction emf in the coils of transformer T1, which creates a positive voltage at terminals 6, 8, 10, 5 and 7 of transformer T1. In this case, current flows through the diodes of the half-wave rectifiers in the secondary circuits VD12-VD15.
With a positive voltage at terminals 5, 7 of transformer T1, capacitors C14 and C6 are charged, respectively, in the anode and control electrode circuits of thyristor VS1 and C2 in the emitter-base circuit of transistor VT1.
Capacitor C6 is charged through the circuit: pin 5 of transformer T1, diode VD11, resistor R 19, capacitor C6, diode VD9, pin 3 of the transformer. Capacitor C14 is charged through the circuit: pin 5 of transformer T1, diode VD8, capacitor C14, pin 3 of transformer. Capacitor C2 is charged through the circuit: pin 7 of transformer T1, resistor R13, diode VD2, capacitor C2, pin 13 of the transformer.
The subsequent switching on and off of transistor VT4 of the autogenerator is carried out similarly. Moreover, several such forced oscillations are sufficient to charge the capacitors in the secondary circuits. With the completion of charging of these capacitors, positive feedback begins to operate between the windings of the autogenerator connected to the collector (pins 1, 19) and to the base (pins 3, 5) of the VT4 transistor. In this case, the self-oscillator goes into self-oscillation mode, in which transistor VT4 will automatically open and close at a certain frequency.
In the open state of transistor VT4, its collector current flows from the plus of capacitor C16 through the winding of transformer T1 with pins 19, 1, the collector and emitter junctions of transistor VT4, parallel connected resistors R14, R16 to the minus of capacitor C16. Due to the presence of inductance in the circuit, the collector current increases according to a sawtooth law.
To eliminate the possibility of failure of transistor VT4 from overload, the resistance of resistors R14 and R16 is selected in such a way that when the collector current reaches 3.5 A, a voltage drop is created across them sufficient to open thyristor VS1. When the thyristor opens, capacitor C14 is discharged through the emitter junction of transistor VT4, resistors R14 and R16 connected in parallel, and open thyristor VS1. The discharge current of capacitor C14 is subtracted from the base current of transistor VT4, and the transistor closes prematurely.
Further processes in the operation of the autogenerator are determined by the state of the thyristor VS1. Opening it earlier or later allows you to regulate the rise time of the sawtooth current and thereby the amount of energy stored in the transformer core.
The power module can operate in stabilization mode and short circuit mode.
The stabilization mode is determined by the operation of the UPT on transistor VT1 and thyristor VS1. At a mains voltage of 220 V, when the output voltages of the secondary power supplies reach rated values, the voltage on the winding of transformer T1 (pins 7, 13) will increase to a value at which the constant voltage at the base of the transistor VT1, where it is supplied through the divider R1-R3, becomes more negative than at the emitter, where it is completely transmitted. Transistor VT1 opens along the circuit: pin 7 of the transformer, R13, VD2, VD1, emitter and collector junctions of transistor VT1, R6, control electrode of thyristor VS1, R14-R16, pin 13 of the transformer. The transistor current, summed with the initial current of the control electrode of the thyristor VS1, opens it at the moment when the output voltage of the module reaches the nominal values, stopping the increase in the collector current.
By changing the voltage at the base of transistor VT1 with trimming resistor R2, you can adjust the voltage across resistor R10 and, therefore, change the opening moment of thyristor VS1 and the duration of the open state of transistor VT3, i.e., set the output voltages of secondary power supplies.
As the network voltage increases (or the load current decreases), the voltage at terminals 7, 13 of transformer T1 increases. This increases the negative base voltage relative to the emitter of transistor VT1, causing an increase in the collector current and a voltage drop across resistor R10. This leads to earlier opening of thyristor VS1 and closing of transistor VT4, the power supplied to the secondary circuits decreases.
When the network voltage decreases (or the load current increases), the voltage on the transformer winding Tl and the potential of the base of the transistor VT1 relative to the emitter become correspondingly less. Now, due to a decrease in the voltage created by the collector current of transistor VT1 on resistor R10, thyristor VS1 opens at a later time and the amount of energy transferred to the secondary circuits increases.
A significant role in protecting transistor VT4 is played by the cascade on transistor VT2. When the network voltage decreases below 150 V, the voltage on winding T1 with pins 7, 13 is insufficient to open transistor VT1. In this case, the stabilization and protection device does not work and the possibility of overheating of the VT4 transistor due to overload is created. To prevent the failure of transistor VT4, it is necessary to stop the operation of the autogenerator. The transistor VT2 intended for this purpose is connected in such a way that a constant voltage is supplied to its base from the divider R18, R4, and a pulsating voltage with a frequency of 50 Hz is supplied to the emitter, the amplitude of which is stabilized by the zener diode VD3. When the network voltage decreases, the voltage at the base of transistor VT2 decreases. Since the voltage at the emitter is stabilized, a decrease in the voltage at the base causes the transistor to open. Through the open transistor VT2, trapezoidal pulses from the diode VD7 reach the control electrode of the thyristor, opening it for a time determined by the duration of the trapezoidal pulse. This stops the generator from working.
Short circuit mode occurs when there is a short circuit in the load of secondary power supplies. In this case, the module is started by triggering pulses from the trigger device (transistor VT3), and turned off using thyristor VS1 according to the maximum collector current of transistor VT4. After the end of the trigger pulse, the device is not excited, since all the energy is consumed by the short-circuited circuit.
After the short circuit is removed, the module enters stabilization mode.
Pulse voltage rectifiers connected to the secondary winding of transformer T1 are assembled using a half-wave circuit.
The VD12 diode rectifier creates a voltage of 130 V to power the horizontal scanning module. The ripples of this voltage are smoothed out by capacitor C27. Resistor R22 eliminates the possibility of a significant increase in voltage at the rectifier output when the load is turned off.
A 28 V voltage rectifier is assembled on the VD13 diode, designed to power the vertical scanning module. The filter at its output is formed by capacitor C28 and inductor L2.
The 15 V voltage rectifier for powering the ultrasonic sounder is assembled using a VD15 diode and a C30 capacitor.
The 12 V voltage used in the control unit, color module, radio channel module and vertical scan module is created by a rectifier using diode VD14 and capacitor C29. A compensation voltage stabilizer is included at the output of this rectifier. It consists of a regulating transistor VT5, a current amplifier VT6 and a control transistor VT7. The voltage from the output of the stabilizer through the divider R26, R27 is supplied to the base of the transistor VT7. Variable resistor R27 is designed to set the output voltage. In the emitter circuit of transistor VT7, the voltage at the output of the stabilizer is compared with the reference voltage at the zener diode VD16. The voltage from the collector VT7 through the amplifier on the transistor VT6 is supplied to the base of the transistor VT5, connected in series to the rectified current circuit. This leads to a change in its internal resistance, which, depending on whether the output voltage has increased or decreased, either increases or decreases. Capacitor C31 protects the stabilizer from excitation. Through resistor R23, voltage is supplied to the base of transistor VT7, which is necessary to open it when turned on and restore it after a short circuit. Choke L3 and capacitor C32 are an additional filter at the output of the stabilizer.
It is often necessary to “power” an amateur radio structure with 12 volts in domestic conditions. Switching power supplies from old third-generation TVs (see Fig. 3.14) of the Slavutich-Ts202, Raduga-Ts257, Chaika-Ts280D and similar models come to the rescue.
Their circuit design is, as a rule, universal; such a power supply will provide an output voltage of 12 V with a useful current of up to 0.8 A.
The output voltage is removed from the contacts:
2 - 135 V (for horizontal scanning);
Contacts 1, 3, 6 of connector X2 (AZ) - as it is designated on the board and in the electrical diagram - are combined and connected to the “common wire”. In Fig. Figure 3.15 shows a schematic diagram of the MP-3-3 power module (similar to the MP-3-1 module used in some models of color TVs of the ZUSTST-61-1 type series).
Rice. 3.14. Type of TV power module
Fig, 3.15. Electrical circuit of the MP-3-3 module
The power cord to the 220 V network is connected to connector XI.
The main difference between these “related” units is in the indicators: the more “fresh” MP-3-3 has an AL307BM LED indicator, and the older version has an INS-1 gas-discharge lamp - through a 135 V power supply limiting resistor. If these indicators after supplying power to a known-good MP-3, they do not light up (which often happens without a connected load), which means that the power module needs to be started artificially. To do this, it is often enough to connect between contacts 1 and 2 (135 V output) an equivalent load - a constant resistor of the MLT-1 type with a resistance of 6.8 kOhm ±30%. After such modification, the pulse generator “starts up”, transformer T1 begins to “sing” quietly, and the power module is ready to operate across the entire spectrum of output voltages. With resistor R27 (designation on the diagram and on the board), you can adjust the voltage at the 12 V output within small limits. There is no need to install additional filtering oxide capacitors (at the output), the shape of the output voltage on the oscilloscope screen has a clear straight line, not burdened by interference.
The most likely cause of failures of these power modules “lies” in a malfunction of the KT838 (VT4) blocking generator transistor. The electrical diagram (Fig. 3.15) shows the values of the control voltages at various points, so it will not be difficult for any radio amateur to repair such a power supply. And the elements for repair can be found in the “bins”, without spending material resources on the purchase of new radio components, as would inevitably have to be done when repairing more compact, but often more “capricious” pulse adapters for modern radio equipment. In this, undoubtedly, “morally obsolete” power modules of the MP-3 type (various modifications) outperform more modern ones, so it is too early to write off the former.
Literature: Kashkarov A.P. Electronic devices for coziness and comfort.
